This invention relates to an improvement in a method for extruding ceramic honeycomb articles, in particular ultra-thinwall ceramic honeycomb articles having 4 mil cell walls or less, and an apparatus for carrying out the same.
Twin screw extruders are commonly employed in the manufacturing of ceramic honeycomb articles. U.S. Pat. No. 4,551,295, herein incorporated by reference in its entirety, is directed at a method of using a twin screw extruder to produce cordierite or cordierite-mullite honeycomb cellular substrates for employment as catalytic converters in automotive engines.
In an effort to meet the demands of ever tighter emission standards, automobile manufacturers are requiring honeycomb cellular substrates with thinner cell walls to provide increased geometric surface area and faster lightoff properties.
A problem which presently exists in the manufacturing of ultra-thinwall honeycomb cellular substrates having 4 mil cell walls or less is die plugging. Die plugging leads to missing webs and non-knitting webs in the substrates thus rendering them unusable and resulting in high production losses. It is believed that large particles and/or contaminants pass through the twin screw extruder screen, enter the die and plug the die slots.
Finer screens may be used to filter out the majority of particles and/or contaminants which would otherwise lead to die plugging. This solution, however, is not ideal because finer screens more readily blind or plug-up and as such cause instability in the extrusion process which is created when the profile of the screen is changed. Plugged screens induce process and production losses. Further the pressures created in a twin screw extruder having a fine screen coupled to or immediately upstream from the die often exceed the pressure limitation of the apparatus causing system failure.
Therefore, it would be advantageous to eliminate the aforementioned problems of die plugging and process instability in the manufacturing of ultra-thinwall honeycomb cellular substrates having a wall thickness of 4 mill or less.
As such, it is an object of the present invention to provide an improvement in a process for making ceramic honeycomb articles, and in particular in a process for making ceramic ultra-thinwall honeycomb articles having 4 mil cell walls or less, without the aforementioned problems.
U.S. Pat. No. 2,336,734 (Jellinek) discloses an extrusion process which involves a preliminary extrusion step to plasticize the batch before final extrusion. Either the entire batch or some portion of the batch is first pre-worked to a desired plasticity in a single screw extruder, formed into strands and then subsequently re-extruded into a final article in another single screw extruder.
U.S. Pat. No. 3,390,216 (Umeya et al.) discloses an extrusion process for ceramic materials which involves pre-shaping a batch in an auger extruder, immediately thereafter passing the batch through a vacuum chamber for degassing, and then subsequently extruding to a final article through a piston extruder while at the same time maintaining the batch in a degassed state.
U.S. Pat. No. 3,745,200 (Geyer) discloses an extrusion process for thermoplastic materials which involves two worm-type extruders. One extruder is used in the conventional way, with the second extruder being employed to receive and regenerate portions of the batch from the conventional extruder back into the conventional extruder at regular intervals.
U.S. Pat. No. 3,888,963 (Orso et al.) discloses a discontinuous extrusion process in which a homogenizing assembly is used to break up and homogenize interfaces between adjacent charges of batch material.
U.S. Pat. No. 5,213,737 (Ford et al.) discloses an extrusion process for powdered materials which involves having one or more gear pumps in succession between the extruder and the die. In one embodiment, the batch is pre-worked in a twin screw extruder and then transferred to a single screw extruder for being fed to the gear pumps and for final forming.
In accordance with one aspect of the present invention, there is provided an improvement in a process of making honeycomb articles, which process utilizes a co-rotating, intermeshing twin screw extrusion apparatus to mix, screen, and extrude a batch of ceramic materials through a die, the improvement which comprises the steps of separating the mixing and screening phase from the extrusion phase, by passing the batch through a first co-rotating, intermeshing twin screw extruder or a mixer extruder to mix and screen the batch, and then directly passing the mixed and screened batch through a second co-rotating, intermeshing twin screw extruder or a pumping extruder to transport and force the batch through a die assembly to produce a honeycomb article.
In accordance with another aspect of the invention, there is provided an apparatus for extruding a ceramic honeycomb article comprising a first co-rotating, intermeshing twin screw extruder or a mixer extruder for mixing and screening a batch of ceramic material, the first twin screw extrusion apparatus comprising a first feed inlet at one end, a first discharge outlet at the other end, and a filtering screen detachably mounted adjacent the first discharge outlet; a second co-rotating, intermeshing twin screw extruder or a pumping extruder downstream from the mixer extruder for transporting and forcing the batch through a die assembly, the pumping extruder comprising a second feed inlet at one end corresponding to the first discharge outlet of the first twin screw extruder and a second discharge outlet at the other end; means for linking the mixer extruder to the pumping extruder; and, a die assembly attached to the second discharge outlet of the pumping extruder for producing a honeycomb article.